This octapeptide is a COOH-terminal fragment of the C3a anaphylatoxin peptide. It results in contraction of rodent ileum and uterus, release of vasoactive amines from rat mast cells, and increases vascular permeability in guinea pig and human skin. Both purified C3a and synthetic C3a , which retainspartially the activity of anaphylatoxin, are shown to interact directly with human lymphocytes.